function Food(el) {
  this.el = el;
  this.len = 20;
  this.foodObj = null;
  this.foodX = 0;
  this.foodY = 0;
}
Food.prototype.create = function() {
  let divObj = document.createElement("div");
  divObj.style.width = this.len + "px";
  divObj.style.height = this.len + "px";
  divObj.style.background = "pink";
  divObj.style.position = "absolute";
  this.foodObj = divObj;
  this.el.appendChild(divObj);
  this.move();
};
Food.prototype.move = function() {
  let foodXMaxNum = this.el.clientWidth / this.len - 1;
  let foodYMaxNum = this.el.clientHeight / this.len - 1;
  this.foodX = Math.floor(Math.random() * (foodXMaxNum - 0 + 1) + 0) * this.len;
  this.foodY = Math.floor(Math.random() * (foodYMaxNum - 0 + 1) + 0) * this.len;
  this.foodObj.style.left = this.foodX + "px";
  this.foodObj.style.top = this.foodY + "px";
};
